1. Field of the Invention
The present invention relates to a magnetic field generator and to a charged particle beam irradiator and, more particularly, to a magnetic field generator for forming a magnetic field by moving a magnetic pair couple in a volume inside a return yoke, and to a charged particle beam irradiator for deflection control of a charged particle beam utilizing a magnetic field formed by the magnetic field generator.
2. Description of the Related Art
A charged particle beam irradiator according to the prior art was disclosed at pages 2055 to 2122, Number 8, Volume 64, 1993, Review of Scientific Instruments, by W. T. Chu, et al. FIG. 1 is a schematic perspective view for explaining an example of the charged particle beam irradiator according to the prior art. A charged particle beam generator 35 generates a charged particle beam and, for example, an accelerator is employed as the charged particle beam generator. A charged particle beam transporter 37 transports the charged particle beam generated by the accelerator 35. For example, a transporter having an electromagnet is employed as the charged particle transporter to transport the charged particle beam generated by the accelerator 35. A charged particle beam deflector 39 deflects the charged particle beam 33 transported by the charged particle beam transporter 37. The charged particle beam deflector 39 may be an electromagnet.
A magnetic field generator 10 generates a magnetic field. The charged particle beam 33 passes through the magnetic field generated by the magnetic field generator. Magnetic poles 3a and 3b form a magnetic pole pair in which the magnetic pole 3a and the magnetic pole 3b are opposite each other.
A coil 1a mis wound around the magnetic pole 3a, and a coil 1b is wound around the magnetic pole 3b. The coils 1a and 1b are connected to a power source (not illustrated), and, by supplying a current from the power source, a magnetic field is formed between the magnetic pole 3a and the magnetic pole 3b. A return yoke 5 is disposed outside the magnetic pole pair 3a and 3b, and the return yoke 5 and the magnetic poles 3a and 3b are one solid unit.
The magnetic field generator 10 is fixed to a toothed gear 21. A toothed gear 22 engages the toothed gear 21. A driver 11, for example, a motor, rotationally drives the toothed gear 22. By driving the motor 11, the toothed gear 22 is rotated, so the toothed gear 21 and the magnetic field generator 10 are also rotated.
The charged particle beam deflector 39 deflects the charged particle beam 33 to move along a rotation axis 29 of the toothed gear 21. The charged particle beam 33 travels along the rotation axis of the toothed gear 21 and enters the magnetic field generator 10.
A magnetic field corresponding to the current flow in the coils 1a and 1b is generated between the magnetic poles 3a and 3b, and a force (Lorentz force) is applied to the charged particle beam passing between the magnetic poles 3a and 3b. This force corresponds to the vector product of the magnetic field and the charged particle velocity. Accordingly, after passing through the magnetic field generator 10, the direction of the charged particle beam is changed (i.e., deflected).
An irradiated object 15 receives the charged particle beam. When the charged particle beam irradiator is applied to a medical treatment appliance, the irradiated object 15 is a human body.
When the charged particle beam is not deflected by the magnetic field generator 10, the irradiation location of the charged particle beam 33 corresponds to the position where the rotational axis of the toothed gear 21 intersects the irradiated object 15. On the other hand, when deflected by the magnetic field generator 10, the irradiated location moves to a position on a straight line along a direction perpendicular to the magnetic field generated between the magnetic poles 3a and 3b. The direction of that movement varies, corresponding to the direction of the current flowing in the coils 1a and 1b, and the magnitude of that movement varies, corresponding to the magnitude of the current flowing in the coils 1a and 1b. By controlling the current flowing in the coils 1a and 1b, the irradiated position may be oscillated along a straight line (such an operation is hereinafter referred to as scanning irradiation).
Further, by rotating the toothed gear 21, the straight line rotates around the rotation axis 29 of the toothed gear 21, so the direction of scanning irradiation also rotates. Therefore, the entire region within a circle 19 on the irradiated object 15 is irradiated by the charged particle beam. The radius of the circle can be changed by varying the magnitude of the current flowing through the coils 1a and 1b.
The charged particle beam irradiator according to the prior art has several problems. Since the magnetic pole 3a, the magnetic pole 3b, and the return yoke 5 are a solid unit in the magnetic field generator, to change the direction of scanning irradiation, all of the magnetic pole 3a, the magnetic pole 3b, and the return yoke 5 must be entirely rotated. However, in using the charged particle beam irradiator as a medical treatment appliance for treating a deep tumor, for example, it is necessary to irradiate the tumor with a heavy charged particle beam, such as a proton beam, a carbon beam, etc., having a high energy (250 MeV-400 MeV per nucleon). In that case, the total weight of the magnetic field generator 10 amounts to several tons.
Accordingly, in the construction according to the prior art, in rotating the magnetic pole pair comprising the magnetic poles 3a and 3b, it is necessary to rotate the return yoke 5 at the same time, together with the magnetic pole pair, which means that a load on the motor 11 is very large. Further, since a large torque motor 11 is required, it is difficult to rotate the magnetic pole pair at a high speed with high precision. Therefore, it takes a very long time to irradiate all of the area within the circle 19.